PRIMARY AND NMR 3-DIMENSIONAL STRUCTURE DETERMINATION OF A NOVEL CRUSTACEAN TOXIN FROM THE VENOM OF THE SCORPION CENTRUROIDES LIMPIDUS LIMPIDUS KARSCH

A crustacean-specific toxin from the Mexican scorpion Centruroides lim pidus limpidus was purified, and its primary sequence was determined, including disulfide bonds. This toxin has 66 amino acid residues and i s stabilized by four disulfide bridges (Cys12-Cys65, Cys16-Cys41, Cys2 5-Cys46, and Cys29-Cys48). A detailed nuclear magnetic resonance struc ture of this protein was obtained using a combination of two-dimension al proton NMR experiments. The NMR parameters that gave 69 dihedral re straints and 418 distance constraints were used in molecular dynamics calculations in order to determine the solution conformation of the to xin. It is composed of a short alpha-helix and a three-stranded antipa rallel beta-sheet. Although the regular secondary structure of this cr ustacean toxin is common to the structural motif of other scorpion tox ins, detailed conformational analysis was performed in order to highli ght structural features that might be responsible for the differential modulation of the toxin on sodium channels of distinct tissues: mamma lian versus crustacean.